Modulation system



SIDEBAND SUPPRESSION /N D8.

Dec. 10, R, Q wlsE 2,224,580

MODULATION SYSTEM Filed Nov. 16 1938 F/a/ ,2 A#7a2 S A L \L L /3 o Arr f/0 U J "P f .f

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A TTORNEV' Patented Bec. 10, 1940 rrr-:D STATES.

PATENT oFFlcE MoDULA'rIoN SYSTEM application November 1s, 193s, serialNo. 240,639

3 claims. (ci. 17a-171.5) j

This invention relates to a method and means of reducing amplitudemodulation of signal waves by any desired degree.

It is often desired to remove noise and other spurious side-bands on acarrier or other single tone supply, to remove amplitude modulation fromsignals in which only frequency modulation is wanted, to reduce thedegree of modulation in a received radio signal, to remove modulationside-bands from a received signal so that its carrier may be used as afrequency reference, to reduce the effective noise in systems nominallycarrying only a single frequency, or to reduce distortion in amodulator.

It is an object of this invention to provide a method and means forattaining such results in a simple and reliable manner.

A feature of the invention comprises the utilization of a negativefeedback connection in which the undesired noise, distortion orside-bands are detected or demodulated, amplified, and remodulated onthe transmission line but with a phase and magnitude to oppose orcompensate for the original undesired component.

In accordance with this invention, a transmission line or system isprovided with a source of signal at one end and any suitable load ortermination at its other end, an amplifier being included in the line. Afeedback connection from the output to the input circuits of saidamplifier comprises a detector or demodulator, an audio frequencyamplifier and a modulator, in the order named, the feedback being ofsuch amplitude and phase that the desired reduction or eliminationoccurs without singing tendency around the feedback path.

A more complete understanding of the invention will be obtained from thedetailed description which follows, taken in conjunction with theappended drawing, wherein:

Fig. 1 is a schematic of a circuit arrangement or system embodying theinvention;

Fig. 2 shows in greater detail the means utilized in practicing theinvention; and

Figs. 3, 4 and 5 show curves illustrating characteristics of a circuitembodying the invention.

Fig. 1 shows schematically a transmission system comprising atransmission path or line i@ having a source S at one end and a suitableload L at its other end. The source S may be, for example, a source ofcarrier waves or other single tone supply including undesired noise orother spurious side-bands, or a source of frequency modulated carrierwaves including unwanted amplitude modulation components, or a source ofreceived radio signals whose modulation exceeds a desired degree, or asource of received signals having modulation side-bands which must beremoved if its carrier is to be used as a frequency reference. Theamplifier A is a high frequency amplifier in the transmission pathintermediate its ends. A feedback connection comprising a balanceddetector or' demodulator D, a low or audio frequency amplifier B, and abalanced modulator M, in the order named, extends between the output andthe input sides of the high frequency amplifier.

The fwaves from the source S pass through the amplifier A and into theload or termination L. The amplifier output is impressed on thedemodulator which produces the demodulated sidebands in the mid-branch,conjugate to the load. These side-band frequencies are amplified by theamplifier B, and are then impressed on the modulator M. The side-bandfrequencies are remodulated on the original input waves, but preferablyare 180 degrees out of phase with the original side-bands and of suchmagnitude as to effectively oppose or compensate for the originalsidebands. The remaining side-bands again pass through the amplifier A,and the process is repeated indefinitely, producing a result which inits broad aspect is identical with that found in negative feedbackamplifiers of the type disclosed in i-l. S. Black Patent 2,102,671,issued December 2l, 1937. It can be readilyy shown that to the firstapproximation, at least, the side-bands are vreduced by the factor 1nl+u l where a has the magnitude and phase found by breaking'the feedbackpathand measuring the v transmission around the'loop as taught in theBlack patent.

The method of design of the loop characteristic is the same as that usedfor the Black type negative feedback amplifiers. Diiculty, if any, willbe encountered in the low frequency path, for the necessary transmissionband may be' several octaves whereas in the amplifier A the transmissionband may be relatively narrow. In some applications, it may be feasibleto eliminate amplifier B and make all the amplifier useful in theforward path. In such a case, if amplifier A has a gain of N decibels,the demodulator should be designed to operate with a carrier N decibelsgreater than the proper carrier for the modulator. Both the modulatorand the demodulator function as linear devices, that is, their output isdirectly proportional to the input.

The circuit arrangement shown in Fig. 2 is representative of an actualembodiment of the invention and comprises the transmission line I0,including a transformer il coupling the high frequency amplifier A andanattenuator I2 to the input terminals I3 of the line. The feedbackconnection comprises the demodulator D, for eX- ing units I4, such ascopper-cuprous oxide rectifier units, a. low-pass filter I5, .an inputtransformer I6, another attenuator I'I, the low frequency amplifier B,an output transformer I8, a high-pass filter I9, and the modulator M,-for example, of oppositely poled asymmetric conducting units 20, suchas copper-cuprous oxide rectifier units. The low frequency amplifier inthe particular case had a sharp cut-off at the low and high frequencies,and the low and highpass filters were used to reduce the gain at thosefrequencies to prevent singing, and enabled as much as twenty decibelsgain around the feedback loop without tendency to singing. Althoughamplifiers of any conventional design may be used, it was found that, ifthe ampliers A and B had degenerative or negative feedback withinthemselves, the phase shift around thefeedback loop was reduced. Again-frequency characteristic'around the feedback loop is shown in Fig.3, the feedback connection having been broken and the measurements madeat X-X.

The carrier Wave for a particular transmission system is frequentlyderived from a. relatively low frequency carrier wave by means of a harmonic producer, the desired carrier wave being a harmonic of the lowerfrequency wave. If the latter contains, as is sometimes the case,troublesome power supply or other undesired side-bands, these side-bandsbecome even more troublesome at the higher desired frequency because theratio of the amplitude ofthe side-bands to that of the harmonicincreases with the order of the harmonic. When high order, highfrequencyl harmonics are used, the requirements for the removal of thelow frequency side-bands become stringent.

A source of '72 kilocycles carrier with two noise side-bands resultingfrom the superpositionof a '700 cycles per second wave on the carrierwas impressed on the input terminals I3 of the line I0, and measurementof the carrier and the side-bands in the load circuit was made by meansof a current analyzer. Curve A of Fig. 4 shows the suppression of theside-bands as a function of the gain around the feedback loop. 'Thetheoretical results to be expected from computations based on themeasurement of gain around the loop is shown by curve B of Fig. 4.

.Closeagreement between the theoretical and actual results is indicated,with only slight discrepancy at" low values of gain. The circuitarrangement described will function for any purpose where it is desiredto reduce theamplitude of double side-bands of a carrier, and it willreduce such side-bands to whatever degree of stable feedback the circuitis designed.

The circuit arrangement described may be used also in the case of acarrier and single side-band. From a theoretical standpoint and to afirst order approximation, it would seem that the side-band should bereduced 6 decibels for very large amounts of feedback and anothersideband added equal in magnitude but 180 degrees Flg. 5. Twofrequencies, Fi, the carrier, and Fz, the single side-band, wereimpressed on the input terminals' I3. With the feedback connection open,'Fa was 18.5 decibels down on Fi, and the other side-band Fs, whereF3=2FiF2 is produced spuriously in the modulator M, was 50 decibelsbelow F1. With the feedback loop closed and the gain around the loopincreased, F2 was reduced by a value approaching 6 decibels and Faincreased to a value approaching F2. Measurement of the demodulateddifference frequency Fz-Fi indicated that it decreased by the value ofl-i-a, showing the two side-bands to be out of phase. For large amountsof feedback, therefore, the demodulated noise is reduced by the amountof feedback or gain around the loop. It follows, therefore, that thecircuit arrangement described will reduce the effects of noise or randomfrequencies in ampliers made to pass a single frequency for, with rareexception, out of phase double side-band noise cannot manifest itself.

The circuit arrangement of Figs. l and 2 can be used to reducedistortion in modulators. Carrier only will be impressed on theterminals I3 of the transmission line I0, while the modulating signal isintroduced in the feedback connection, for example, at the terminalsY-Y. The distortion side-bands are reduced by a process analo gous tothe reduction of distortion in the Black amplifier.

As a demodulator, the circuit arrangement alsoA back connection betweenthe output and the input of said amplifier for limiting the amplitude ofsaid side-band, said feedback connection including, in the order named,a demodulator, an audio frequency amplifier and a modulator, each ofsaid amplifiers having degenerative feedback within itself whereby phaseshift around the feedback loop is reduced.

2. In combination, a transmission line containing an amplifier, a sourceof carrier frequency current at one end and a load at the other end ofsaid line, a feedback path from thev output to the input of saidamplier, said path including a demodulator, an `amplifier and amodulator in theorder named, each of said amplifiers having degenerativefeedback within itself whereby phase shift around the feedback loop isreduced.

3. 'I'he method of reducing thev amplitude of an unwanted component in acarrier frequency wave to be modulated and transmitted over atransmission line, which comprises amplifying said wave, degenerativclyfeeding back a portion of the amplified wave, demodulating the amplifiedwave, amplifying the demodulated wave, ,degeneratively feeding back aportion of the demodulated amplified wave and modulating said amplifieddemodulated wave on the unamplified' carrier frequency wave in suchphase and of such amplitude as substantially to balance out saidunwanted component.

1 RAYMOND O. WISE.

